1. Field of the Invention
The present invention relates to a valve characteristic control apparatus or, in particular, to a configuration of the oil paths of a valve characteristic control apparatus for changing valve characteristics by displacing a camshaft by oil pressure.
2. Description of the Related Art
The valve characteristic control apparatuses for changing valve characteristics are roughly divided into two types. One changes the valve open period itself and the other shifts the valve open period without changing the valve open period. In the former type, the camshaft is moved in axial direction in many cases. The latter type, on the other hand, is often achieved by means of changing the rotational phase of the camshaft with respect to the crankshaft and camshaft is moved by oil pressure in most cases.
An example of the latter type is a valve characteristic control apparatus disclosed in JP-A-8-326515. This apparatus comprises an advance oil chamber formed between a pulley and a camshaft for changing the rotational phase of the camshaft in such a manner as to shift the valve operation phase to the advance side when the working oil is supplied thereto, and a retard oil chamber for changing the rotational phase of the camshaft in such a manner as to shift the valve operation phase to the retard side when the working oil is supplied thereto, wherein the working oil is supplied to the advance oil chamber and the retard oil chamber from a working oil supply control valve arranged in a support member for supporting the camshaft.
The working oil is supplied to the camshaft and an annular advance oil path and an annular retard oil path formed in a sliding portion of the support member for pivotally supporting the cam shaft through the advance oil path and the retard path formed in the pivot member. From the annular advance oil path and the annular retard oil path, the working oil is introduced through another advance oil path and another retard oil path formed in the camshaft to the advance oil chamber and the retard oil chamber, respectively.
The advance oil path in the pivot member and the annular advance oil path communicate directly with each other, and so do the retard oil path in the support member and the annular retard oil path. Therefore, the advance oil path in the support member and the retard oil path in the support member are required to be spaced from each other. As a result, the distance between the annular advance oil path and the annular retard oil path is also increased. The axial length of the engine is thus increased, thereby posing the problem of a deteriorated mountability and an increased weight of the engine.
JP-A-8-246818 also discloses same type of valve characteristic control apparatus. One embodiment shown in JP-A-8-246818 has an angled in-support-member retard oil path which requires complex machining. In other embodiments an in-support-member retard oil path is made as a groove on a bearing cap for easier machining. However, in these embodiments sufficient sealing is not obtained because of a small gap between the groove and an annular advance oil path on a cam shaft. Therefore, a bearing with a larger width or with a larger flange is required for a sufficient sealing.
This problem is not limited to the valve characteristic control apparatus of the phase-shift type, but is also shared by any valve characteristic control apparatus having the same structure for supply and discharge of the working oil.